The object of the invention relates to the technical field of magnetic sensors including an encoder component moving close to a detection cell, and broadly speaking, adapted for locating at least one angular position.
The object of the invention more particularly relates to making a sensor, the encoder of which is equipped with a series of North poles and South poles mounted in an alternating way.
The object of the invention finds a particularly advantageous application in the automotive field where this sensor may for example be used within the framework of ignition functions.
In the preferred field above, it is known how to apply an adapted magnetic sensor for measuring the intensity change of a magnetic induction field when a ferromagnetic encoder provided with field perturbation units, moves past a detection cell. The detection cell such as a Hall effect or magneto-resistive probe, for example, delivers a periodic sinusoidal signal. The detection cell is associated with a level comparator with hysteresis, such as a Schmitt trigger, in order to obtain clear-cut transitions of the output voltage for distinct values of the magnetic induction according to whether it changes by increasing or by decreasing.
In order to form a velocity detection sensor, it is known how to make an encoder provided with teeth laid out in a regular way and in high numbers for improving the resolution of such a sensor. An enhancement to this sensor is known which consists of making an encoder formed by a multipolar magnetic ring provided on its circumference with alternating North poles and South poles, regularly spaced out according to a given pitch.
In order to allow at least one position to be determined, which for example corresponds to the ignition top dead centre of a cylinder, it is known how to make a mark on the magnetic encoder. Thus, it is for example known how to suppress two teeth on the cog wheel. In the solution applying an encoder with alternating North and South poles, either suppressing several magnetic poles by leaving an empty space, or replacing one or more poles of a given sign with one or more poles of an opposite sign, may be contemplated. A so-called irregular or singular pole having a magnetization with a sign opposite to the sign of both of its adjacent poles on the one hand, and a different spacing relatively to the spacing pitch of the other poles on the other hand are thereby achieved.
In order to achieve good accuracy in measurement, notably as regards the detection of the irregular pole, French Patent FR 2 757 943 teaches how to make an encoder comprising, for each irregular pole, means for correcting the value of the magnetic field generated by the irregular pole, so that the signal delivered by the passage of the poles neighboring said irregular pole, is symmetrical relatively to the zero value of the magnetic field.
By applying such an encoder, it is possible to obtain at the output of the detection cell of the sensor, a magnetic signal with constant period, as regards the regular poles. As a result of this, good accuracy of the thereby conducted measurements notably for locating the irregular pole, is achieved.
If the technical solution described in this patent gives satisfaction in practice, under certain operating conditions, a, large range variation of the width of the delimited gap between the encoder and the measurement cell was seen on the one hand, and a not insignificant side shift was seen on the other hand between the plane of rotation of the encoder and the axis of the measurement cell, which is detrimental to the accuracy of the measurements.
Also, U.S. Pat. No. 4,866,381 describes a position sensor including an encoder provided on its circumference with alternating North poles and South poles and mounted in order to move past a measurement cell. Such an encoder includes a magnetic singularity formed by a series of North and South poles having a determined pitch adapted for stabilizing the magnetic signal delivered by the passage of the poles adjacent to this singularity and inside this singularity. The ideal signal of this position sensor includes, upon passing the singularity, a succession of high and low states, consecutive to the passages through zero of the corresponding magnetic signal at different poles of the singularity.